1. Field of the Invention
The present invention relates generally to a mobile communication system and, more particularly, to a device and method for transmitting a preamble of an access channel in a CDMA (Code Division Multiple Access) mobile communication system.
2. Description of the Related Art
The term “access channel” as used herein refers to all channels transmitted by a transmission party requesting a reception party to establish a link for the channels. That is, the access channel refers to all channels on which a known signal such as a preamble is transmitted prior to transmission of a message. The access channel used herein is not specifically limited to an access channel as conventionally defined in related art mobile communication systems. For instance, the access channels include reverse access channel (R-ACH), reverse common control channel (R-CCCH), and reverse dedicated access channel (R-DACH).
In order to accurately receive a signal from the transmission party, the reception party has to be in synchronization with the signal transferred from the transmission party. This sync acquisition is a very important factor which determines capability of the CDMA communication system.
In a mobile communication system, a mobile station acquires synchronization with a signal received from a base station according to a specified sync acquisition procedure beginning at the moment the mobile station turns power on. The mobile station maintains the synchronization through a sync tracing procedure that lasts until the mobile station turns the power off, so that it can resume communications with the base station at any time. In the sync acquisition procedure, the mobile station uses a reference signal such as a pilot channel. The reference signal is transmitted to an unspecified mobile station within a cell area that is controlled by the base station. The base station can transmit the reference signal continuously while the system operates, since the reference signal is transmitted to an unspecified mobile station. Because the reference signal is previously scheduled between the base station and the mobile station according to a certain engagement, the mobile station can receive signals from the base station whenever the power is switched on, by tracing the reference signal and acquiring synchronization with it.
In contrast, the sync acquisition procedure at the base station does not begin at the moment the mobile station switches the power on. The reason for this is that the mobile station inhibits unnecessary signal transmission and establishes a transmit link only at the moment a message or data to transmit exists, thereby minimizing power consumption at the mobile station and reducing interference on the base station. This link establishing procedure includes the sync acquisition procedure in which the base station acquires synchronization with a signal received from the mobile station.
For efficient sync acquisition, the mobile station transmits a preamble PA shown in FIG. 2 to the base station for a defined time interval prior to sending a message or data. The term “preamble” as used herein refers to a signal previously scheduled between the base station and the mobile station. In most mobile communication systems, the beginning of the preamble transmission interval is determined by a fixed system parameter, or can be selected at the mobile station based on the transmission time available as determined by visual information in the system. Here, the visual information is obtained from a base station signal acquired after the mobile station switches the power on. A receiver at the mobile station detects the preamble at the beginnings of all preamble transmission intervals estimated from the visual information of the system and acquires synchronization. Upon detection of the preamble, the base station performs sync acquisition and sync tracing procedures to receive a message following the preamble.
FIG. 1 is an illustrative diagram of an access channel transmitter at the mobile station in accordance with prior art.
Referring to FIG. 1, a preamble generator 120 generates a preamble as indicated by the reference numeral 210 of FIG. 2. An amplifier 122 raises transmission power for a reverse pilot channel (R-PICH) in the preamble interval to be higher than transmission power for the reverse pilot channel in an access channel message (message capsule) interval. A selector 124 is used to select the preamble interval and the message transmission interval. The selector 124 selects the output of the amplifier 122 at the beginning of the preamble interval and selects a non-amplified signal at the ending of the preamble interval. This operation of the selector 124 is performed once per one access channel. But, there is no need to separately use the selector 124 in a case where the amplifier 122 converts amplification gain from pilot gain (“Gp”) to “1” in the preamble interval and the message transmission interval. That is, the amplifier 122 sets the gain to “Gp” at the beginning of the preamble interval and sets the gain to “1” at the ending of the preamble interval. The gain of the amplifier 122 is set only once during one access channel interval. A mixer 110 multiplies orthogonal codes (+1, −1, +1, −1) by a transmission symbol for the access channel so as to distinguish the access channel from the reverse channel. The access channel is not transmitted in the preamble interval but transmitted to the base station at the beginning of the message capsule interval, i.e., after the ending of the preamble interval. An amplifier 130 determines a transmission power ratio of the reverse pilot channel to the access channel in the message capsule interval. A complex spreader 140 spreads a signal for the reverse pilot channel, a signal for the access channel and PNi (Pseudo-random Noise in-phase) and PNq (Pseudo-random Noise quadrature phase) sequences. Among the signals spread at the complex spreader 140, a real signal is applied to a filter 150 and an imaginary signal is applied to a filter 152. The filters 150 and 152 are pulse forming filters for the transmit signal. Amplifiers 160 and 162 amplify the outputs of the filters 150 and 152 to a strength which is transmittable through an antenna. Mixers 170 and 172 multiply the output signals of the amplifiers 160 and 162 by a carrier and convert them to radio frequency (RF) band signals. A π/2 phase converter 180 maintains a phase difference between a carrier multiplied by the I (In-phase) channel and a carrier multiplied by the Q (Quadrature phase) channel at 90 degrees. A combiner 190 combines the outputs of the mixers 170 and 172 and outputs the combined signals to the antenna.
Now, reference will be made in connection with FIG. 2 as to an example of a signal structure transmitted on an access channel at the mobile station in accordance with the prior art.
Referring to FIG. 2, the mobile station transmits a preamble 210 to the base station for a defined period of time (e.g., N*1.25 ms) prior to the message capsule interval. The mobile station then sends a reverse pilot channel with transmission power lowered to the strength as indicated by the reference numeral 280. The preamble and the reverse pilot channel may be generated from an identical sequence generator or different sequence generators. The reverse pilot channel is used for reverse link channel estimation or sync tracing procedure and may include forward pilot information. The reason that the preamble 210 is transmitted with higher transmission power than is used for the reverse pilot channel is to facilitate preamble detection and sync acquisition at the base station. That is, the higher transmission power for the preamble 210 is used to increase detection probability and reduce miss probability and false alarm probability. A message capsule 280 contains a reverse channel message and data to be transmitted to the base station.
A problem with the related art access channel transmitting method is that the preamble transmission interval is relatively long and the preamble is transmitted with relatively high transmission power though there is no message to transmit, thereby increasing interference on the reverse link channel. Therefore, there is a need for a method for minimizing interference on the reverse link channels as well as increasing detection probability for the preamble.